Harry Becks has a lot to answer for. Thanks to his graphic wizardry, confused tourists still think London Underground connects equidistant stations; that Heathrow is only a short journey from Acton; and that the Thames runs in nice, regular loops. If only.

Becks was the genius who dreamt up the modern London Underground map. With its colour-coded horizontal, vertical and 45-degree diagonal lines, each marked with interchanges, it looks like a glorified circuit diagram. This is scarcely surprising: Becks was an electrical draftsman.

As Mark Ovenden reveals in his delightful, oddball Metro Maps of the World, the origin of Becks's creation can be traced to urban rail transport's early days when lines were recorded merely by etching them on to existing city maps. The proliferation of railways soon made such representations incomprehensible.

So Becks stripped away every piece of surface detail (apart from the Thames) and displayed only essential data - which station follows which and where lines meet. Physical distances were ignored, so Acton, halfway between Leicester Square and Heathrow, appears three times closer to the airport than the West End.

The result has endured for 70 years, unchanged, with the exception of new Tube lines in the 1970s and 1990s. As Ovenden states: 'It is difficult to imagine an image more ingrained into the very psyche of a population.'

And what was good for London soon proved to be a boon for Boston, Osaka, Lisbon, Vienna and most other cities. Each has its local idiosyncrasies, of course - the map of Oporto's new system looks like a swastika, Glasgow's exciting circle of 15 stations is still too confusing for most citizens on a Saturday night, while Tokyo's looks like an accident at a noodle factory. They are all great maps, though they are really just footnotes to Becks. Posh isn't the word for him.